In a process of manufacturing a semiconductor device, matching a positional relation between patterns formed through different processes at high precision has a very great impact on the operative properties of a device. Therefore, a technology to measure the positional relation at high precision is important in a process of manufacturing a semiconductor device, and miniaturization of a device is made progress in these days, required precision continuously becomes strict.
In general, in a match measuring technology used when manufacturing a semiconductor device, an alignment mark belonging to a lower layer is adjacent to an alignment mark belonging to an upper layer and their relative positional relation is measured. Since there is a need to penetrate the upper layer so as to measure the alignment mark of the lower layer, an optical microscope is used for measuring.
Therefore, the alignment mark is formed in a pattern of which one side is 100 nm or larger and which can be observed at a corresponding wavelength band, or smaller patterns are densely enumerated, thus an alignment mark of a size recognizable with the optical microscope is formed.
On the other hand, due to progress in miniaturization of a lithography technology, a pattern of a state-of-the art device is formed in a pattern which is 100 nm or smaller in minimum dimension. For this reason, a situation that deviates between dimensions of a pattern involved in an operation of a device and an alignment mark configured to measure the positional relation of the upper and lower layers of the device pattern occurs.
In a lithography process, since a dimension of a pattern on a photomask is smaller than a wavelength of a light source radiated from an exposure unit to a photomask, a light radiated to a pattern diffracts and forms an image on a wafer through an optical system of the exposure unit. However, the smaller the dimension of the pattern is, the larger the angle of diffraction becomes, in principle. Therefore, as a result that lights radiated to and diffracted from respective patterns of different dimensions diffract at different angles of diffraction, the lights pass through different parts of a lens, thus the patterns image-formed on the wafer have different impacts due to a local distortion of a lens.
From the description above, it is expected that an image formation state of light penetrating a lens can be different between an alignment mark and a device pattern and such a trend will become conspicuous in the future as a miniaturization of a device pattern progresses.